Rotary offset printing presses having rotatable cylinders and removable impression and blanket sleeves are generally well known in the art. Such presses typically operate at very high speeds and are capable of printing a high quantity of material in a relatively short period of time. A continuous web of paper passes between a pair of rotating blanket cylinders which print images on opposites sides of the paper web. Each blanket cylinder is in contact with a plate cylinder having an impression sleeve which has been inked and dampened and which transfers the images to the blanket cylinder for printing onto the web in a manner well known in the art.
In order to change the printed material, such as when a newspaper, magazine or brochure is switched to a different edition, the plate cylinder is moved away from its adjacent blanket cylinder, the impression sleeve on the plate cylinder is removed, and a different impression sleeve is installed. When the changeover process is complete the press is ready for the next printing run.
Many times, such changeovers occur with great frequency, such as when small jobs are being printed. Unfortunately, the process of changing the impression sleeve is very labor intensive and time consuming, and thus there is considerable down time for the press. Typically, each cylinder in the press is mounted for axial rotation between a pair of spaced apart side walls. The impression sleeves are mounted to the cylinders, and fit so snugly that the sleeves are held in place by friction. In order to move the sleeve relative to the cylinder, compressed air is forced between the inner surface of the sleeve and the outer surface of the supporting cylinder. The cushion of air expands the sleeve slightly, and allows the sleeve to slide relative to the cylinder. Thus, in order to install or remove the impression sleeve from the plate cylinder, the plate cylinder must first be disconnected and removed from the side walls. Thereafter, a new impression sleeve is placed on the cylinder in the same manner and the rotatable cylinder is reinstalled in preparation for the next printing run. As outlined above, this is a very time consuming process and seriously undermines the cost effectiveness of the press when the press is being used on relatively small jobs.
A number of approaches have been attempted in order to decrease the changeover time between printing runs. For example, one approach as disclosed in U.S. Pat. No. 4,807,527 is to provide a releasable bearing on one end of the cylinder shaft. Removal of the bearing assembly creates an access hole in the press side wall and exposes one end of the cylinder shaft so that the impression sleeve can slide off the shaft through the access hole. The other end of the shaft is elongated, and during the changeover process the elongated portion of the shaft abuts an auxiliary shaft which is put in place for temporary support.
Similarly, U.S. Pat. No. Re. 34,970 discloses a pivotable bearing which swings away to free up one end of the cylinder for the removal of the sleeve, and also discloses a cylinder supported by a pair of linearly retractable bearings, and finally a cylinder mounted to a swivel on one end and having a retractable bearing on the other.
Unfortunately, in addition to other shortcomings, each of the prior art devices requires some means of temporary cylinder support in order to effectuate the changeover of the impression sleeve. In addition, each of the prior art devices requires that at least one of the bearing assemblies be completely disconnected from the cylinder shaft, and thus, neither of these approaches provides a cost effective solution to the problems outlined above.
Another problem with prior art printing presses is that all of the rotating cylinders in the machine are mechanically connected to a single drive shaft system, which creates a number of inherent drawbacks. For example, all of the rotating cylinders and rollers in a printing press are typically connected to a common drive system, which consist of an extensive collection of drive shafts, gearboxes and pulleys, all of which is designed to spin all of the cylinders in the press at the same peripheral speed. Because all of the cylinders must have access to the same drive system, the placement of the cylinders relative to each other is severely constrained, which adds to the difficulty in changing impression sleeves on the plate cylinders. Moreover, on large presses there is noticeable lash in the drive system, which causes registration and vibration problems, both of which negatively impact print quality.
Still another problem is the difficulty in maintaining acceptable print quality when longer cylinders are used. For example, because the outer end of a cantilevered cylinder may deflect, it is difficult to maintain even printing pressure along the length of the cylinder. Such a problem is of course exacerbated when longer print cylinders are used. Uneven cylinder pressure causes web wrinkling and web migration.
Accordingly, there exists a need for a rotary offset printing press having cantilevered cylinders which permit fast replacement of the impression sleeve and which do not require temporary support during changeover. There also exists a need for self-driven cylinders which reduce or eliminate drive line lash and which also improve registration and overall system performance. Such cylinders will preferably be supported in such a manner that print quality is maintained even when relatively long cylinders are employed.
There also exists a need for a system for supporting cylinders, whether cantilevered or not, in such a manner that the pressure between the cylinders along their length can be made substantially uniform.